Self-starting magnetic escapement mechanisms



July 26, 1960 c. F. CLIFFORD SELF-STARTING MAGNETIC ESCAPEMENTMECHANISMS Filed June 6, 1956 3 Sheets-Sheet 1 INVENTOR ATTORNEYS July26, 1960 c. F. CLIFFORD 2,946,183

SELF-STARTING MAGNETIC ESCAPEMENT MECHANISMS Filed June 6, 1956 3Sheets-Sheet 2 INVENTOR ATTORNE Y5 July 26, 1960 c. F. CLIFFORD2,946,183

SELF-STARTING MAGNETIC ESCAPEMENT MECHANISMS Filed June 6, 1956 3Sheets-Sheet 3 FIG. 7 (/voN- OPERATIVE) (/va/v- OPERATI r) INVE N TORUnited States Patent 0 SELF-STARTING MAGNETIC ESCAPEMENT MECHANISMSCecil Frank Clifford, Bath, England, assignor to Horstmann CliffordMagnetics Limited, a British company Filed June 6, 1956, Ser. No.589,782

Claims priority, application Great Britain June 14, 1955 3 Claims. (Cl.58-116) This invention relates to magnetic escapement mechanisms and isparticularly, though not exclusively, applicable to such mechanisms asdescribed in my United States Patent 2,690,646.

In magnetic escapement mechanisms, as distinct from the previously knownmechanical escapements such as the English lever or Swiss cylindricalescapement, the complementary rotational and oscillatory movements arecoupled by a magnetic loc usually between the escape wheel and thecontrolling oscillator instead of by mutual physical engagement of suchparts. The parts providing such magnetic lock are an undulating magnetictrack (continuous or interrupted) and complementary pole salients, thetrack and pole salients being arranged for relative longitudinal andtransverse movement, the longitudinal movement (i.e. in the direction ofthe neutral axis of undulation of the magnetic track) being provided byrotation of the escape wheel and the transverse movement being providedby an oscillatory system having a natural frequency of oscillation,whereby, under normal working conditions, the magnetic pole salientsrelatively follow the undulations of the magnetic track and the saidnatural frequency of oscillation controls the speed of rotation.

In the more usual construction of a magnetic escapement, the magneticpole salients are carried by the oscillator in jaw-like mannerrelatively embracing the magnetic track, and oscillatable transverselyof the neutral axis of such magnetic track, the magnetic flux pathbetween the pole salients passing through the track. A magnetic force,therefore, exists which tends to resist displacement of the magnetictrack from between the pole salients in any manner which causes changeof reluctance of the magnetic flux path. Such magnetic force preventsrelative jumping movement of the pole salients across the gap betweenadjacent undulations of the magnetic track and therefore compels thepole salients to follow the undulations of the track as above explained.Such magnetic force is termed a magnetic lock.

One problem connected with magnetic escapements as above described is tomake them self-starting, and the primary object of the present inventionis the solution of such problem.

Experience has shown that it is comparatively easy to construct amagnetic escapement of the kind referred to which will work reliablyunder normal conditions, so that some problems, such as regulation of,and isochronous compensation for, the escapement have already been thesubject of earlier applications for patent in connection for example,with spring-driven timepieces where it is necessary to provideregulating means to adjust the escapement for correct time-keeping, and

Patented Jul 26, 1960 to ensure isochronism within the normal amplitudeof oscillations varying with the range of operating torque applied tothe escape wheel by the spring between the fully wound and unwoundconditions thereof.

Self-starting for magnetic escapements has not hitherto beensatisfactorily obtained, the torque normally required to start rotationof the escape wheel having usually to be so great as to cause the escapewheel to accelerate too rapidly so that it does not come under thecontrol of the oscillator. Self-starting, however is in itself, aseparate problem, the object of the present invention, and for itscomprehension it is necessary to consider the controlling factors. Theexpression self-starting is used herein to mean not merely thecommencement of rotation of the escape wheel but the impulsing therebyof the oscillator in such manner that the rotation of the escape Wheelis automatically brought under the control of the oscillator for normalactuation of the escapement.

In the first place, the relative undulation of the magnetic track andthe proportions and disposition of the pole salients relative theretomust be such that, when the oscillator comes to rest, with falling ofthe torque applied to the escape wheel below that necessary to maintainoscillation, the escape wheel must also be held at rest, just as with amechanical escapement. Otherwise, the escape wheel could run-away at aspeed uncontrolled by the oscillator. In other words, the magnetic lockwhich, under normal working conditions prevents the relative jumping ofthe track undulations by the pole salients inevitably provides a reposeresistance (i.e. the resistance to movement of the escape wheel when theoscillator is stationary) which must be broken by escape Wheel torquebefore rotation can commence to make the escapement self-starting.

The only factor which can prevent run-away in a magnetic escapement isthe magnetic lock, coupling the relative rotary and oscillatorymovements, and this lock is provided by the hollow or notch of eachtrack undulation and the air gap resistance provided thereby to preventrelative jumping of the track by the pole salients. At normal runningamplitude the available air gap resistance will always far exceed therupture force available from the escape wheel. On starting, however, therepose resistance which is provided in part at least by the magneticlock has to be overcome to move the escape Wheel from one rest positionto the next, i.e. the available magnetic lock must be broken. Afterinitial rupture, and until normal running conditions are established,the magnetic lock provides only negligible resistance to acceleration ofthe escape wheel. This is because displacement or impulsing of the polesalients is at first negligible due to the wide divergence between thefrequency of impulses from the escape wheel and the natural frequency ofthe oscillator. Myklestadt in his book entitled, Vibration Analysis hasshown that on an oscillator such as the one with which we are concerned,there is no appreciable magnification factor, i.e. magnification ofdisplacement of the oscillator, until the escape wheel has acceleratedto a speed at which the impulses from the undulations of the track comewithin of the natural frequency of the oscillator. The magnificationfactor even at is only four times that of static deflection butthereafter it rises steeply until synchronism is reached. If the escapewheel accelerates beyond synchronisation, the magnification factordecreases even more steeply and at it falls below unit value.

Consequently, there is only a narrow range of speed of the escape wheelon each side of synchronised speed (hereinafter termed the criticalimpulsing speeds) during which the escape wheel accelerating afterinitial starting rupture of the magnetic lock can impulse the oscillatorto an amplitude which will provide an air gap resistance of sufficientsize to establish a controlling magnetic lock between the pole salientsand the magnetic track. The only magnetically based possibility of aselfstarting magnetic escapement is therefore to have a low reposeresistance so that a small torque will effect initial starting ruptureof the magnetic lock and initiate movement with a resultant relativelyslow acceleration of the escape wheel, and at the same time to have ahigh magnetic impulsing factor which will be a function of the magneticlock, so as to induce a controlling amplitude of oscillation before theescape wheel can be accelerated too far through the critical speedaforesaid.

It is a further object of the present invention to obtain the longestpossible running from a given available amount of driving energy, sothat the relation between length of run and self-starting is clearlydependent also upon similar conditions which contribute towards reliableselfstarting.

Accordingly to the invention the pole salients and complementa'rymagnetic track of the magnetic escapement are so shaped and proportionedas to provide a low repose resistance with initial displacement of theoscillator, and the strength of the restoring force for the oscillator,the mass of the oscillator and the moment of inertia of the escape wheelare so proportioned that a torque applied to the escape wheel sufficientto overcome the repose resistance can accelerate the escape wheel to thecritical impulsing speeds and whilst its speed of rotation is within therange of such speeds the magnetic track will induce an amplitude ofoscillation of the pole salients sufficient to establish a controllingmagnetic lock.

By the present invention the advantageous result of a self-startingescapement is achieved capable of reliable self-starting over areasonably wide range of available starting torques.

If the starting torque is of the magnitude most frequently encounteredin the normal operation of a spring motored clock, the effect of time onthe power requirements is less significant, and self-startingcalculations can be based upon the force theoretically transmitted fromthe escape Wheel to the oscillator during the initial movement of theescape wheel.

Of particular importance to the present invention, it was discoveredthat a very light escape wheel, having a very low moment of inertia,accelerated too rapidly through the critical impulsing speeds and didnot allow time within the critical speeds aforesaid for inducing acontrolling amplitude of vibration in an oscillator having its mass andthe stillness of its mounting reed Within proportions generallyrecognized as suitable on account of other considerations. On the otherhand, there are limitations to increasing the moment of inertia of theescape wheel or to decreasing the mass of the oscillator, and stiffnessof the mounting reed.

A further important object of the present invention is the proportioningof the magnetic track and the pole salients to provide in effect astarting track adapted to produce a low repose resistance and anefiFective impulsing of the oscillator during the acceleration aforesaidof the escape wheel. The magnetic track is desirably so designed thatthe sinusoidal path of the projection of the pole salient may be incomplete engagement with the magnetic track at any amplitude exceptsmall amplitude vibrations when there will only he partial engagement ofthe sinusoidalstarting track.

In the development of the present invention, it was discovered that theeffectiveness of an oscillating magnet in controlling the accelerationof an escape wheel was influenced by the extent to which magneticinterenga'gem'ent between the salients and the magnetic track wasmaintained. The magnetic tracks of the present invention maintaincontinuous but sometimes incomplete magnetic engagement, i.e. the magnetpoles overlap the magnetic track a little, not only when the mechanismoperates at standard speed, but also when it accelerates from repose, assuch displacement provides impulsing of the oscillator.

It 'was established that particularly beneficial results were achievedby providing a magnetic track having an imaginary non-undulating medianpath of a width constituting a significant portion of the width of thepole salients. Thus, if the pole salients were fixedly held at theircentered position and if the escape wheel with its magnetic track wererotated past such fixedly held pole salients, the centre of'the magnetwould always maintain magnetic interengagement with the centre of thesaid imaginary median path. H V V v In normal operation of theescapement mechanism of the present invention, the pole salients are notfixedly held at their centered position, but instead are displacedslightly toward the teeth on alternate sides of the magnetic track whenthe escape wheel begins to move. The progressive magnitude of suchdisplacement has already been explained. A preferred embodiment of thepresent invention employs an imaginary starting track inscribed withinthe magnetic track and consisting of a slightly undulating wavy pathhaving undulations which are several times smaller than half the fullamplitude of the oscillator. 'The starting track may be defined as animaginary track of substantially constant width, the boundaries of whichare defined on alternate sides by the notches between the teeth of themagnetic track. Such a starting track would have its inner boundariestangential to the boundaries of the median path aforesaid and such innerboundaries would not embrace all the fringing fiux on that side of thesalient magnetic poles when in the repose position. Embodied within thestarting track is the imaginary non-undulating median path aforesaid.The starting track consists'of said median path and marginal portionshaving starting apertures or notches alternately on either side of it,which starting apertures are the portions of the operating track formingthe undulating magnetic track. A salient magnet pole projected over theentire median path and deflected sufiicie'ntly that the projection ofone edge would coincide with an edge of the median path would bepositioned with the projection of its opposite edge coinciding or nearlycoinciding with the outermost edge of the undulating path, that is, justclearing the'aperture. In other words, the starting track which maybeimagined as having uniform width is usually slightly wider than the polesalient, and consists of the median path with successive undulations onopposite sides thereof. The crosswise distance of the starting trackbetween the successive apices on the successive undulations is severaltimes smaller than half the full amplitude of the oscillator. Thestarting track is adapted to s'ta'rt' the oscillator into oscillationfrom the repose condition and is characterised by a sufficiently smallchange of cross-sectional area of magnetic interengagement (i.e.suiiiciently small undulations) to provide low repose resistance tomovement. Hence, moderate starting torques can break the magnetic lockto move the escape Wheel from repose to its first quarter-cycleposition. The starting track also employs suificiently large undulationsso that the magnetic shift imparts a sufficient force to the oscillator'to overcome the centering force and moment of inertia, of theoscillator, thereby initiating a very sm'all'v'ibration during the firstcycle movement of the rotatable member. The controlling magnetic lockbetween the escape Wheel and the oscillator is dependent on the air gapenergy at each undulation and is necessarily low for low amplitudes ofoscillation owing "to the fiat characteristics of thestarting'wavytrack, but such lock must transmit su-filcient energy to the oscillatorto absorb some of the escape wheel energy which would otherwise beavailable for too rapid acceleration of the escape wheel and preventre-building of the magnetic lock within the critical speed range aboveexplained. Thus the mechanism prevents the escape wheel from racing ordeveloping a speed so far beyond its normal or standard speed as to bebeyond the regulatory control of the available magnetic lock which isre-established as synchronisation is reached.

The starting track has a band width which is usually slightly more thanthe width of the pole salient. The median path has a width normallyslightly smaller than that of the pole salient but due to the eiiects ofmagnetic fringing a median path slightly wider than the pole salient cansometimes be effective in impulsing the oscillator.

In shifting from complete engagement with one starting tooth to completeengagement with another starting tooth during a half cycle operation,the magnetic pole salient is subjected to a calculable force. If theundulations are relatively large compared to the cross-sectional area ofthe pole salient, the locking action of the magnetic interengagementbetween the pole salient and the magnetic track prevents the initiationof rotation of the escape wheel unless an excessively high startingtorque is applied. If the undulations are excessively small, the forcestransmitted to the oscillator by the initial rotation of the escapewheel are insufiicient to impulse the oscillator, quickly enough to theregulatory amplitude within the critical speed of rotation, therebypermitting the escape wheel to develop sufiicient acceleration to attaina speed beyond the control of the oscillator.

The present escapements are self-starting when the escape wheel is firstmoved by any one of a Wide range of starting torques.

It is an object of the present invention to so control the size of thestarting teeth or undulations relative to the cross-sectional area ofthe pole salients that the initial starting torque of the escape wheel,will be suiiicient to initiate and increase vibration of the polesalient during the relatively uncontrolled acceleration of the escapewheel particularly as its speed approaches the critical speed aforesaid,to an amount so at all times to transmit enough energy to the oscillatorto prevent excess storage of enengy in the escape wheel beyond thatcontrollable by the available magnetic lock as that becomesreestablished on attaining synchronisation of impulsing and oscillation.

It is an object of the present invention to so control the width of themedian path that this adventageous result of self-starting is achieved.

In the accompanying drawings:

Fig. 1 is a perspective View of one example of a self-starting magneticescapernent made in accordance with the present invention;

Fig. 2 shows an alternative embodiment of a self starting magneticescapement according to the invention;

Fig. 3 is a schematic diagram in developed form from Fig. 1 showing thearea relationships aliecting the selfstarting operation;

Fig. 4- is a schematic edge View related to Fig. 3;

Fig. 5 is a perspective view of an apparatus having a stationarymagnetic track and made in accordance with the present invention;

Fig. 6 is a schematic view in developed form of the track and showingthe successive positions of a pole salient in a self-starting operationembodying a magnetic track similar to Fig. 5

Fig. 7 is similar to Fig. 6 but shows a magnetic track which isinoperative and incapable of achieving self-starting because the smallmagnitude of a normal operating torque is insuflicient to overcome therepose resistance due to the magnetic lock attributable to the largestarting teeth or undulations and narrow median path; and

Fig. 8 is similar to Figs. 6 and 7 but shows a magnetic track which isinoperative and incapable of achieving proper self-starting by reason ofthe small impulsing force imposed upon the pole salient due to the smallmagnetic locking action attributable to the small undulations and widemedian path.

Referring now to the drawings, and particularly to Figs. 1 and 3, thereis shown an oscillator 1. Advantages result from mounting the oscillatorin such a manner that the axis of oscillation passes through the centreof gravity of the oscillator and in providing the oscillator with acentering force powerful enough to be shockresistant and to operateuniformly without regard to the position of the oscillator with respectto gravitation, as explained in said United States Patent 2,690,646.

An escape wheel 3 is mounted for rotation so that it is influenced bypole salients 5 of the oscillator mag net 1, which is carried by aflexible reed 1a secured to a post lb of non-magnetic material at theend of a bracket is of non-magnetic material. The escape wheel 3 isprovided with outwardly extending magnetic teeth 8 and with inwardlyextending teeth 9.

Particular attention is called to an imaginary undulating starting track18 shown dotted having a path width slightly greater than the width ofthe pole salients 5 as explained below in more detail. The startingtrack 18 is a slightly undulating path which is adapted to start thepole salients to oscillate from their reposed condition upon the initialmovement of the escape wheel. The overall width of the starting track 18between the apices on opposite sides of the undulating path is such thatit constitutes only a small portion of half the full amplitude of theoscillator. The maximum intended amplitude of the oscillator is lessthan that which would result if the magnet vibrated to a position at theextremity or" the magnetic teeth 8. Thus the distance which correspondsto half the full amplitude of the standard oscillation of the mag at isseveral times greater than the full width of the starting track. Animaginary median path 26 of a non-undulating nature is shown inscribedWithin the starting track.

it will be noted that the median path 20 is of smaller width than theoverall width of the starting track. Moreover, the median path 20 isnarrower than the width of the magnet pole salients 5 by an extent whichcorresponds to the width of a starting tooth or overlap of an aperture.The centered salient with its magnetic fringe overhangs the median pathon each side by one half the radial width of a starting tooth.

In Fig. 2, there is shown another embodiment of a selfstarting magneticescapement mechanism. An oscillator 2G1 is mounted so that the axis ofoscillation passes through the centre of gravity of the oscillator. Polesalients 2G5, 295 are in magnetic engagement with a dual-escape wheel263 provided with outer and inner magnetic teeth 2.08 and 299respectively. During the normal operation 0: the magnetic escapement,the escape wheel 233 is permitted to rotate only at a speed controlledby the natural period of vibration of the pole salients 2&5 by reason ofthe magnetic lock between the pole salients 2% and the undulatingoperating track 219. When the magnetic escapement mechanism of Fig. 2 isat the repose position, as shown in Fig. 2, and the escape wheel isgiven an initial rotary movement, the magnet pole salients 205 are urgedinto a small amplitude vibration by the imaginary starting track 218.The progressive nature of such urge and the negligible magnitude ofdisplacement until the normal or critical speed is approached has beenfully explained above. Inscribed within the starting track 218 is animaginary non-undulating median path 220 having a width which isnarrower than the pole salients 23:; by the extent of starting teeth ofthe starting track 213.

An embodiment of the invention particularly suited for a schematicillustration of the self-starting principles is shown in Fig. 5.Magnetic pole salients 5% of an oscillator 501 are in magneticengagement with a stationary magnetic track member 521. The magnetictrack member 521 is a cylindrical ring constructed of soft iron or othermagnetic pole salients 505 oscillate up and down, i.e.

parallelto the vertical axis. The pole salients 595 are on oppositesidesof the stationary magnetic track 521. The braking forceattributable to. the magnetic lock between the pole salients 505 and themagnetic track 521 is sufficient that when the escape wheel is operatingat its predetermined speed, consistent with thenormal period ofvibration of the oscillator, and the relationship thereto of themagnetic track 521, effective speed control is achieved. Because of suchmagnetic lock, great power is required to accelerate the escape wheelbeyond its standard speed. When no force is applied to the escape wheel,it continues to rotate at its standard speed until the amplitude of theoscillator becomes too small to allow the momentum of the wheel to carryit past the magnetic lock of the next tooth of the starting track. Theoscillator is thus capable of controlling the speed of an escape wheel,by reason of the magnitude of the forces of the magnetic lock betweenthe pole salients and the magnetic track. The power necessary toaccelerate the escape wheel from its predetermined speed is much greaterthan the power required to operate theescape wheel at its predeterminedspeed because such higher speeds are attainable only by overcoming themagnetic lock between the vibrating pole salients and the magnetictrack, i.e. forcing relative jumping by the pole salients of the air gapbetween adjacent teeth. Obviously for larger amplitudes the availableair gap reluctance is larger.

In Fig. 6 there is a schematic diagram in developed form showing aportion of a magnetic track member 621 similar to the member 521 of Fig.but as if of infinite diameter. The pole salients 605 at repose maintaina position which is substantially centred on the magnetic track 621, asindicated at 605a, thus being centred over the median path 620. When theescape wheel is rotated counter-clockwise by an initial starting torqueof suitable power, the magnet is moved to the right according to Fig. 6thereby bringing into action a starting tooth 623. If there issufficient power to accelerate the escape wheel from the reposecondition, there is suflicient power to deflect the pole salient a smallamount from its reposed position. If the starting tooth 623 is of a sizewhich is neither too large nor too small relative to the width of thepole salient, then the initial power for deflecting the magnet can besuflicient, While the escape wheel speed is within the criticalimpulsing speeds, to re-establish the magnetic lock after it has beenbroken for starting rotation of the escape wheel.

The deflecting force imposed on the pole salient by the quarter cyclemovement above described of the escape wheel is a function of thestrength of the magnetic field, the permeability of the magnetic track,the distance between the magnetic poles, the related factors. However,assuming that the only variable concerns the undulating shape of themagnetic track and the relative shape and size of the pole salients,certain principles are applicable. Thus, the starting teeth orundulations must be of suflicient size that the pole salient is urged todeflect a small amount during the first quarter-cycle of movement. Insome of the magnetic escapements heretofore available, difliculty hasbeen encountered by reason of a repose resistance. aforesaid. If themedian path has Zero width, so that the starting teeth are as wide asone-half of the width of the pole salient, a relatively strongdeflecting force is imposed on the magnet in moving from a reposedposition to a quarter-cycle position but a relatively stronger torque isrequired for the rupture of the magnetic lock to enable the rotation ofthe escape wheel to commence. The escape wheel thus can be locked atrepose if the initial starting torque is too small or can require astarting torque so great as to produce excessive acceleration of theescape wheel after initial rupture of the magnetic lock. Suchaninoperable condition is diagrammatically shown in Fig. 7, in which thestarting teeth 723 are relatively large, and in which the median path720 is less than of the width of the pole salient 705.

In Fig. 8 there is shown a schematic diagram of a magnetic track 819having a median path 820 which is about twice the width of the polesalients thereby rendering the starting teeth 823 nonefiective fromrepose. The starting torque required to break the magnetic lock will benegligible and the impulsing force imparted to the pole salient 805during the initial rotation of the escape wheel will be insufiicient toestablish controlling vibration of the magnet, thereby permitting thestarting torque on the escape wheel to accelerate the escape wheel to aspeed beyond that for which it was intended, and after which there isnegligible impulsing of the oscillator at its naturalfrequency and,hence, little energy is spent and the oscillator is ineffectiverestraining and regulating the speed of the escape wheel.

In the development of the present invention, it was establishedthat thewidth of the median track should be at IeasL30% and not more than 120%of the width of the pole salient in order to provide a self-startingescapement.

Data relating to the area relationships are shown in Figs. 3 and 4. Thusthe median path GH is narrower than the width of the pole salient AD.The magnet salient ABCD is shown in an unstable position of equilibriumfor the escape wheel where it has a maximum downward urge, but theresultant displacement is so small that it is shown centered over themedian path GH. In moving from such position to an intermediate reposeposition the escape wheel would go through a quarter-cycle rotation andthe pole salients would take up a position between two teeth one on oneside and one on the other side where.

a larger portion of the track is covered by a projection of the polesalients. The change of projection (in making this movement) of thecentered salient ABCD onto the magnetic track is proportional to thearea ABEF, assuming that the magnet is fixedly held in centeredposition. In normal operation, however, the rotation would initiate thedeflection of the magnet away from the aperture between its jaws. Indetermining the amount of force necessary to move the escape wheelduring the first quarter-cycle of the operation, however, it isconvenient to consider only the forces involved in the magnetic lock,assuming that none of the force is utilised for the deflection of themagnet, which in any case is negligible for the initial acceleration ofthe escape wheel. On this basis, the starting torque must besuflicientto bring about a change in the area ABEF of the flux member inthe magnetic field.

In Fig. 4, the thickness of the escape wheel MM and the distance AA (orBB) between the pole salients are diagrammatically represented. Thesedistances are signifi cant because they affect the energy necessary tobring about a reduction of the area of the flux member in the magneticfield. Expressing the change of area (a) in square centimeters, theenergy (e) in crgs, the thickness (t) of the escape wheel incentimeters, and the intensity of the magnetic field (B) inlines offorce per square centimeter or gauss, the following equation shows therelationships:

in tB As an example of the calculations, reference can be made toestimating the appropriate change of area of projection of a magnetictrack on a centered pole salient in moving from a repose position to aquarter-cycle position. If the escape wheel had a diameter of 1centimeter, and if it revolved revolution per quarter cycle, and if theinitial torque (T) was 20 dyne cms., the energy (e) would be 11' crgs,as indicated.

Thus, the area difference would be about 0.0126 mm.

If, in the pole salient ABCD, the distance AB was about 0.5000 mm. andthe distance AD was about 0.4500 mm. the distance AF would be about0.0250 mm. in order to achieve the area (a) which could be modified bythe available 3.1416 dyne cms. of initial torque on the escape wheeland, hence 3.1416 ergs of energy in moving ,4 revolution. Such a torquecould rupture the magnetic lock in moving the escape wheel from areposed position to a quarter-cycle position adjacent a tooth as shownin Fig. 3. If the initial starting torque was only half of that employedin the calculations, then the repose resistance would be sufficient toprevent movement of the escape wheel. If the change in area ABEF weretwice as great as calculated, then twice as much initial torque would benecessary for overcoming the repose resistance. However, if the areaABEF were eight times that of the numerical example, then the medianpath would be so narrowed that the advantageous magnetic track of thepresent invention would not be attained. Only if the median path is atleast 30% of the width of the pole salient, and not more than 120% ofsuch width, can reliable self-starting be achieved.

It will be noted that the midpoints of the successive teeth are spacedlongitudinally from each other a distance corresponding to several timesthe length of the pole salient, whereby the centering forces of theoscillator can urge the displaced pole salient toward the center beforethe starting track can advance to the center of the next tooth. Thestarting track is not only of sufiiciently small amplitude, but is alsosutficiently elongated to assure initial vibration of the oscillatorduring the acceleration of the escape wheel and to assure theestablishment of a controlling amplitude of vibration by the escapewheel as it.

reaches the critical speeds aforesaid.

In one example of the invention the following are the essential detailsfor a spring driven clock:

(1) The oscillatory system has a weight of 0.3 grms. approximately and anatural frequency of 100 cycles per second.

(2) The supporting spring reed for the oscillatory system has a ratingof 20 grs. per mm. displacement of the poles.

(3) The magnet in open circuit between the pole salients has a strengthof approximately 4,000 lines per sq. cm.

(4) The magnet gap is 0.5 mm. Wide with an escape wheel (of Mumetal)0.25 mm. thick, hence there is an air gap of 0.125 mm. between eachmagnet pole and the escape wheel.

(5) The moment of inertia of the rotary system is 0.1 X gm. cm. sec.

(6) The median path has a width of 0.2 mm.

(7) The thickness of the magnet is 0.35 mm.

(8) Hence the ratio of the median path to pole thickness is A magneticescapement having the essential details above given was found to requirea torque of .05 mm. grs. to start rotation of the escape Wheel andprovided good self-starting for a spring-driven clock-Work mechanismdesigned to give a maximum torque of 0.2 mm. grs. falling to about 0.1mm. grs. over a running period of 24 hours. The escapement was in factcapable of selfstarting with a torque as low as .05 mm. grs. and as highas 0.22 mm. grs.

i 0 A slow motion film has beentaken of an experimental escapement, theanalysis of which is as follows:

Percent of Percent of proper Time Teeth Portions Revs. per speed speed,(secs) moved of revs. see. final corrected for film speed error 0 0 0 00 .131 )4 Mo .19 3.8 4.2 .154 1 A0 1.08 21.6 24 .l78 2 M0 208 41.6 46.2l7 4 it 2.56 51. 2 56 .244"--. 6 340 3.7 74 81 .2680. 8 340 4.16 83.292 .290 10 $5 4. 55 91 400 20 1 4. 55 91 100 At .290 seconds, /2 rev.movement, there was still no visible amplitude of the magnet. At .400seconds, one rev. movement, the amplitude was 0.010 inches or one thirdof the normal amplitude (.030 inch).

The above analysis shows that, until the rotor reaches substantially itsnormal speed after half a revolution, there is no appreciable impulsingof the oscillator, but that by that time and thereafter the impulsingmust have been sufficient to establish a magnetic lock between the rotorand oscillator as the rotor ceases to accelerate while the amplitude ofoscillation builds up to normal.

I claim:

1. A self-starting escapement comprising a rotatable member, anoscillator member urged toward a centered position by forcessubstantially proportional to the amplitude of displacement from thecentered position, mag netic pole salients carried by one of saidmembers, a magnetic track presented by the other of said members, meansassociating the pole salients and said magnetic track to regulate thestandard operating speed of the rotatable member by the normal period ofvibration of the oscillator member, said magnetic track comprising amedian path constituting a significant portion of the width of the polesalients and comprising an undulating starting track whereby the powernecessary to move the rotatable member from a stationary magneticallylocked position is significantly less than the power exerted by thequarter cycle rotation of the rotatable member relatively to theoscillator by reason of the starting track providing a deviation of themagnetic track from the median path between points on the magnetic trackcorresponding to such quarter cycle rotation, the path of magneticinterengagement including said median path and said other member havingstaggered teeth on either side thereof, the spaces between which provideapertures, said undulating starting track including small startingundulations on opposite sides of the median path of the magnetic trackto initiate small amplitude displacement of the oscillator first to oneside and then to the other While the rotatable member initially rotatesan amount corresponding to one normal cycle of vibration of theoscillator, and said magnetic track providing at successive oppositelydisposed teeth a shift of projection from the fixedly centered polesalient upon the magnetic track from one side to the other of the medianpath more than 1% but less than 70% of the Width of the pole salient.

2. A self-starting escapement comprising a rotatable member, anoscillator member urged toward a centered position by forcessubstantially proportional to the amplitude of displacement from thecentered position, magnetic pole salients carried by one of saidmembers, a magnetic track presented by the other of said members, meansassociating the pole salients and said magnetic track to regulate thestandard operating speed of the rotatable member by the normal period ofvibration of the oscillator member, said magnetic track comprising amedian path constituting a significant portion of the width of the polesalients and comprising an undulating starting track whereby the powernecessary to move the rotatable member from a stationary magneticallylocked position is significantly less than the power exerted by thequarter cycle rotation of the rotatable member "rela tively to theoscillator by reason of the starting track providing a deviation of themagnetic track from the median path between points on the magnetic trackcorresponding to such quarter cycle rotation, the path of magneticinterengagement including said median path and said other member havingstaggered teeth on'either side thereof, the spaces between which provideapertures, said undulating starting track including small startingundulations on opposite sides of the median path of the magnetic trackto initiate small amplitude displacement of the oscillator first to oneside and then to the other while the rotatable member initially rotatesan amount corresponding to one normal cycle of vibration of theoscillator, and the path of a centered pole salient on the oscillator,at positions corresponding to successive oppo- 12 sitely disposed teeth,projecting on the median path portion and a starting tooth portion, saidstarting tooth portion constituting at least 1% but not more than 50% ofthe area of the pole salient. I

3. An escapement according to claim 2 in which the median path of thetrack is 30% to 90% of the width of the pole salient whereby the pole isheld always in magnetic engagement with the track and tends to vibratewhen the rotatable member rotates an amount corre- 10 sponding to twoteeth on the magnetic track.

References Cited in the file of this patent UNITED STATES PATENTS2,654,989 Nichols a Oct. 13, 1953 2,690,646 Clifford a Oct. 5, 1954FOREIGN PATENTS 183,020 Austria Jan. 15, 1955

